In return-to-zero (RZ) coding the frequency spectrum of a coded signal will include a strong peak at the clock frequency. The presence of such a strong peak means that clock recovery can be achieved simply by filtering at the clock frequency.
In non-return-to-zero (NRZ) and similar coding systems there is no such peak in the frequency spectrum of the coded signals. Indeed, the frequency spectrum of such signals will show a dip to near zero power at the clock frequency. Consequently, with NRZ and similar coding systems, it is not possible simply to filter out a component at the clock frequency in order to perform clock recovery.
The most common form of digital coding in which the coded data contains no significant component at the clock frequency is NRZ coding. For ease and clarity of description, the present invention is described herein with reference to NRZ data and NRZ coding. It is to be understood, however, that the invention is also applicable to coding systems which are not, strictly-speaking, NRZ, but which nevertheless produce data streams which do not contain a significant component at the clock frequency, and for which clock recovery is required. Hence, throughout the description, unless the context clearly requires otherwise, references to NRZ systems should be taken to include systems which are not NRZ but to which the present invention is nevertheless applicable.
For processing NRZ coded data signals, it is necessary to generate a clock signal synchronous with the data signals. In processing NKZ optical signals, it is conventional simply to convert the NRZ optical signals into NRZ electrical signals, the NRZ electrical signals then being processed electronically to generate a clock signal. If the signal processing is being carried out preparatory to onward transmission of optical NRZ signals, further conversion is required to generate NRZ optical signals from the processed NRZ electrical signals. Such conversion, which is currently employed in regenerators and at exchanges, is disadvantageous for several reasons. Thus, in regenerators, the complexity and power consumption of the requisite electronic circuitry are limiting factors. Moreover, both in exchanges and regenerators, the use of electronic processing imposes, in the long-term, an upper limit on the optical bit-rate used.
The need for this electronic processing also stands in the way of the realisation of one of the next major advances sought in optical telecommunications, namely all-optical routing.